1. Field of the Invention
The present invention relates to a control device, a control method, and a storage apparatus for causing a head to fly above a rotating storage medium so as to read/write data and particularly relates to a control device, a control method, and a storage apparatus for controlling the clearance between a head and a storage medium surface to a predetermined target clearance by varying a protrusion value by thermal expansion that occurs along with power distribution to and heating of a heater provided in a head.
2. Description of the Related Arts
Conventionally, the clearance between a recording surface of a magnetic disk and a head has to be reduced in order to realize high recording density of a magnetic disk apparatus, and the clearance generated by flying of the head is designed to be constant by utilizing the pressure (positive pressure/negative pressure) of the air that flows into the space between a slider in which the magnetic head is disposed and the magnetic recording medium. In order to improve the recording density, the distance between the head and the medium has to be reduced. Therefore, the clearance of the head has been reduced year by year along with increase in the density of the recording density, and the clearance of 10 nm order has been realized recently. In a conventional head, since the clearance is maintained merely by the pressure of air, variation in the clearance generated among heads, for example, variation in the clearance due to the environmental temperature, mounting variation generated upon mounting of magnetic heads, and manufacturing variation generated among heads cannot be corrected. Therefore, the clearance has to be designed by the conditions that take the above described variations into consideration. However, since variations of the clearances are present among heads within common differences of mechanisms, there is a problem that the clearance cannot be set smaller than the range of common differences when medium contact is taken into consideration. Moreover, since a medium rotates at a high speed in a magnetic disk apparatus, collision with minute protrusions of the magnetic disk surface readily occurs when the clearance of the head is reduced; and, when the medium and the magnetic head are brought into contact with each other, there is a problem that the life of the apparatus cannot be satisfied due to wear of the head and contamination (dust) or the like that is generated when the head is brought into contact with the medium. Therefore, recently, as described in Patent Document 1, there has been proposed a method in which the variations in the protrusion value (TPR value) caused by the phenomenon in which the head flying surface protrudes in the direction toward the magnetic disk due to temperature increase of a write head (thermal protrusion: TPR) are measured in test processes or the like and retained on the magnetic disk, and the clearance is managed for each head by using the data. Furthermore, there have been also proposed methods like those of Patent Documents 2 and 3 in which a heater is built in a head, and the clearance between the head and the recording surface of a magnetic disk is controlled by utilizing the protrusion phenomenon that is caused by thermal expansion of the head flying surface along with power distribution to the heater. In Patent Document 2, a constant element temperature is maintained by varying the electric power, which is applied to an electrically conductive film provided in a head, with respect to the apparatus temperature or increase in the element temperature caused by recording/reproduction, thereby maintaining a constant clearance between the element and the recording medium. In Patent Document 3, a clearance-increasing heating device, which causes a part of an air bearing surface of a head to expand and protrude by heating so as to increase the distance between a recording/reproducing element and a magnetic disk surface, and a clearance-reducing heating device, which causes another part of the head air bearing surface to expand and protrude by heating so as to reduce the distance between the recording/reproducing element and the magnetic disk surface, are provided in the head, and the clearance is corrected so that reproduction can be performed without causing collision upon activation of the apparatus.    [Patent Document 1] Japanese Republished Kohyo Patent Publication No. 2002-037480    [Patent Document 2] Japanese Patent Application Laid-Open (kokai) No. 2005-071546    [Patent Document 3] Japanese Patent Application Laid-Open (kokai) No. 2005-276284    [Patent Document 4] Japanese Patent Application Laid-Open (kokai) No. 2003-273335
However, the above described methods, in which a heater is provided in a head to control the clearance between it and a magnetic disk recording surface, have a problem that the clearance upon recording cannot be measured even when the clearance upon reproduction can be measured although the clearance has to be measured both upon the reproduction performed by electric power distribution to the heater and the recording performed by electric power distribution to the heater, and the heater electric power that controls the clearance to an optimal target clearance upon reproduction and recording cannot be precisely determined. More specifically, in conventional clearance measurement upon reproduction, the relation that the smaller the clearance, the more the amplitude of a head read signal increases is utilized, thereby obtaining an optimal clearance by adjusting the amount of the heater electric power distribution, for example, so that the error upon reproduction has a predetermined value. Meanwhile, upon recording, in addition to expansion and protrusion of the head flying surface that is caused by electric power distribution to and heating of the heater, there is heat generating expansion that is caused when a recording current flows through a recording coil of the head; therefore, in the method in which the clearance is measured by performing reproduction and error detection after data is recorded, the clearance upon recording wherein the clearance is further reduced cannot be clarified, and precise clearance control by heater electric power distribution is difficult. In the clearance measurement that is based on error detection of a head read signal, there is a problem that the error detection accuracy obtained through recording/reproduction is dependent on the recording performance of a recording element and reproduction output power of a reading element, error detection sensitivity is different among heads, and precise clearance measurement is difficult to perform.